Dental milling machine method

Abstract

A dental milling machine is provided with a tool which is guided in a chuck. A workpiece arm is mounted opposite the tool to be able to be moved at least in the direction of the spindle axis. With a control device it can be moved in relation to the workpiece, especially by means of a drive motor of the workpiece arm. The workpiece arm has an abutment element, whereby the control device moves the tool and the abutment element towards each other until they abut each other. The control device can be used to detect abutment of the abutment element with respect to the tool, in particular with respect to its tip, in particular by decelerating the (relative) movement of the tool and the abutment element towards each another, upon contact of the tool and the abutment element. This position may be signaled as a reference position and, especially may be stored.

Claims

1. A method for determining a reference point or position between a workpiece and a tool of a dental milling machine by determining a position of a tip of the tool of the dental milling machine, the dental milling machine used for milling the workpiece and having a control device, a tool clamped in a chuck, and a workpiece arm having an abutment element at a location which is spaced apart from the workpiece, by a predetermined value, comprising moving the workpiece arm with the abutment element translationally in relation to the tip of the tool, wherein a movement of the workpiece arm is performed by a drive motor, detecting the movement of the workpiece arm by an encoder which is coupled to the drive motor, wherein the encoder, together with the control device, acts as a slip sensor and are configured to detect if drive slip of the drive motor occurs or not, as a result of abutment of the abutment element at the tip of the tool, and wherein the first occurrence of a drive slip is evaluated by the control device as an indication of said abutment.

2. The method according to claim 1, wherein the abutment element is spaced apart from the workpiece at an axis of the workpiece, and wherein the indication of said abutment determines the reference position of the tool.

3. The method according to claim 1, wherein the workpiece arm including the abutment element is moved towards the tip of the tool, which is clamped in a fixed tool spindle, by a drive motor configured as a stepping motor, and step loss of the stepping motor is used as a slip detection.

4. The method according to claim 1, wherein the drive motor is operated at a reduced drive current when approaching the tool.

5. The method according to claim 1, wherein the abutment element is laterally mounted on the workpiece arm, spaced apart from the axis of the workpiece by a predetermined amount and also spaced apart from the workpiece.

6. The method according to claim 1, wherein the control device directs relative movement of the abutment element and the tip of the tool as a linear, translational movement to and from the tool.

7. The method according to claim 1, wherein the control device directs the workpiece arm including the abutment element towards the tip of the tool such that the abutment element always contacts the tip of the tool at the same location, even during tool replacement.

8. The method according to claim 1, wherein the dental milling machine is configured as a 5/0 machine tool comprising five axes of movement for the workpiece arm.

9. The method according to claim 1, wherein, for reference point determination of the contact point between the abutment element and the tool, the control device switches the encoder of the drive motor of the tool arm from open loop to closed loop.

10. The method according to claim 1, wherein combination of the control device and the encoder is configured as a slip sensor, through which the control device detects or signals the contact of the abutment element with the tool tip when slip occurs during movement of the workpiece arm towards the tool tip.

11. The method according to claim 1, wherein the milling tool comprises a code, and wherein a milling tool detection device specifically detects the milling tool on the basis of the code, and the detection is stored in the control device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The associated drawings are intended for a better understanding of the embodiments of the invention. They are to illustrate the embodiments and to explain the principles and concepts of the invention in connection with the description.

(2) Other embodiments and many of the advantages mentioned will arise from the drawings. The elements illustrated in the drawings are not necessarily to scale, wherein:

(3) FIG. 1 is a perspective schematic representation of the dental milling machine in accordance with a preferred embodiment of the invention, in the processing state;

(4) FIG. 2 is an illustration of the dental milling machine shown in FIG. 1, but devoid of the workpiece, illustrating the abutment element according to the invention;

(5) FIG. 3 is an illustration of the abutment element according to the invention for recording a length and/or position of the milling tool according to the preferred embodiment of the invention;

(6) FIG. 4 is a block diagram showing the connection of various components in the dental milling machine in accordance with the invention; and

(7) FIG. 5 is an illustration of the workpiece holder according to the invention.

DETAILED DESCRIPTION

(8) In the figures of the drawings, equal reference numbers denote equal or functionally equivalent elements, components or units, unless otherwise indicated.

(9) FIG. 1 shows a schematic representation of the dental milling machine according to a preferred embodiment of the invention.

(10) The dental milling machine 1 comprises a tool 14, which is guided in a chuck 12. The dental milling machine 1 also comprises a robot arm, herein referred to as workpiece arm 2, which is provided with a chuck 2a, into which a workpiece 3 with a corresponding workpiece holder 4 can be inserted, as shown in FIG. 5. FIG. 5 shows the workpiece 3 attached to workpiece holder 4, which workpiece holder 4 is inserted into chuck 2a.

(11) Specifically, the chuck 12 can be rotated, but is otherwise fixedly mounted to the machine. The milling tool 14 is clamped into the chuck 12 via its invisible shaft.

(12) The workpiece arm 2 is mounted opposite to the tool 14 such that it can move at least in the direction of a spindle axis A1. In addition, the dental milling machine 1 has a control device 18. The control device 18 is configured to detect the relative position of the workpiece arm 2 in relation to the milling tool 14 via an encoder 19, which is coupled to a drive motor of the workpiece arm 2, which mechanics are discussed below in reference to FIG. 4.

(13) The milling tool 14 has a ring 20 formed therein. The ring 20 is arranged at a distance from the tip 14a of the milling tool 14 and serves as a stop when clamping into the chuck 12.

(14) The ring 20 is mounted to the milling tool 14 at the transition portion to its shaft 23. The shaft 23 has a flat surface with an accuracy of especially 15 μm, in relation to the distance D to the tip of the milling tool. The tip 14a of the milling tool preferably is equipped with a diamond. Alternatively, other tips 14a, such as metal tips, can be provided.

(15) A milling tool detection device 22 is provided. The milling tool detection device 22, for example a camera, detects the respective milling tool 14 by means of a code, especially an optical two-dimensional code, such as a Data-Matrix-Code. The milling tool is measured in advance and the distance D thereof between the ring 20 and the tip 14a of the milling tool 14 is stored, especially also in the control device 18. Furthermore, the dental milling machine 1 comprises a sensor or temperature-measuring device 24. The control device 18 comprises a memory for an expansion table, which stores the experimentally determined expansion of the milling tool based on a temperature change, and indicates the precise position of the tip of the milling tool based on this change in length.

(16) The ring 20 especially comprises a groove through which a tool replacement arm, for example the tool changer or gripper holder 29 on the workpiece arm 2, can engage into the milling tool 14, thus removing the milling tool from the chuck. In the state of being mounted therein, the tool can be guided in front of the camera 22 so as to perform the above-described milling tool detection.

(17) The substantially annular surface 26 surrounding the chuck also comprises a plurality of nozzles 26a. On the one hand, the nozzles 26a have the function of directing a jet of liquid to the milling tool 14, enabling wet processing of a workpiece by the milling tool 14. In addition, the nozzles 26a have the additional function of delivering compressed air to an environment through the nozzles 26a. By expelling compressed air through the nozzles 26a, the ring 20 or the milling tool 14 can be cleaned from impurities.

(18) FIGS. 1 and 2 show a schematic representation of a contact or abutment element 28 attached to the workpiece arm 2 to measure a length of the milling tool according to the preferred embodiment of the invention. The abutment element 28 is configured as an essentially flat metal sheet and will be described in more detail below. It is mounted adjacent to or together with the gripper holder 29 for a milling tool 14.

(19) The gripper holder 29 for the milling tool 14 has a first holding element 16b and a second holding element 16c. The first holding element and the second holding element are configured such that the first holding element 16b or the second holding element 16c can engage into the respective milling tool 14 in the region of one of the grooves of the ring 20, the groove of the ring 20 of the milling tool 14 being able to engage into the first holding element 16b or the second holding element 16c.

(20) The first holding element 16b and the second holding element 16c are arranged offset with respect to one another on the workpiece arm 2, such that a milling tool 14 is inserted, for example, in the holding element 16c, another milling tool, which is arranged in the chuck 12, can be removed from the chuck 12 by the first holding element 16b, and, in a subsequent working step, the milling tool 14 inserted in the holding element 16c can be transferred into the then unloaded chuck 12.

(21) In the position shown in FIG. 1, the abutment element 28 is at the top of the workpiece arm 2. The workpiece arm 2 can spatially be moved along five axes, and, among others, can also be rotated so that the abutment element 28 can be located at the bottom (FIG. 3).

(22) The abutment element is attached at the side of the workpiece at 2. FIG. 2 shows a state of the dental milling machine according to the invention, which allows a reference position to be determined.

(23) In this state, it is not required for the workpiece arm 2 to carry a workpiece 3.

(24) Reference point determination refers to the determination of the actual position of the tip 14a of tool 14 in relation to the workpiece arm in which the workpiece 3 will subsequently be clamped.

(25) Even if the workpiece arm 2 herein is shown as being movable and the tool 14 is shown with its spindle motor being fixedly mounted in space, it is to be understood that kinematically inverse mounting is also possible without leaving the scope of the invention.

(26) Basically, the reference position is determined so that the workpiece arm 2 is lowered with the abutment element 28 facing downwards in the direction of the arrow 31 until the abutment element 28 contacts the tool 14.

(27) The contact position is also shown in FIG. 3. The abutment element 28 preferably consists of hardened sheet steel and is mounted with high precision on the workpiece arm 2. The tip 14a of tool 14 meets the abutment element 28 at a planar point.

(28) It is to be understood that, if required, a second reference detection can also be performed by detecting the wear of the tool 14 at its tip by inclining the abutment element 28.

(29) The system preferably is detected via a slip sensor. Encoder 19, which is coupled to the drive motor for the vertical movement of the workpiece arm 2, is used for this purpose.

(30) The encoder 19 emits signals for moving the workpiece arm 2 in the direction of the tool tip 14a.

(31) For reference position detection, the drive motor now is supplied with a lower drive current than usual. This enables smooth downward movement. Encoder 19 detects the linear movement and sends the corresponding signals to control device 18.

(32) When the abutment element 28 is in contact with the tool 14, the encoder 19 only emits those signals, which correspond to a downward movement of the workpiece arm 2.

(33) The drive motor tries to move the abutment element 28 downwards, but does not succeed in doing this as it is in contact with tool 14.

(34) Accordingly, there is a slip between the control signal for the drive motor, also emitted by control device 18, and the control signal for the actual movement, corresponding to the signals from encoder 19.

(35) The first time a drive step is skipped, i.e. the first slip, is now detected and evaluated by the control device to indicate abutment between the tool and the abutment piece and will be signaled accordingly.

(36) The principle of the encoder 19 is further illustrated in FIG. 4. Encoder 19 is to detect transitions between (e.g., dark) lines being printed onto a (e.g., lightly colored, pale or white) disc of the encoder. When one motor of the workpiece arm 2 is turning due to respective control commands being sent from the control unit 18, the encoder disc is turning the same way since it is fixedly mounted on the motor axis 25. When turning, the lines printed on the encoder disc pass the light source and sensor 21. Sensor 21 is detecting the change of the reflected light due to the continuous change between the dark line and light background color of the enclosure disc. These transitions between light and dark are used to derive a degree of the motion of the encoder disc and thus of the motor axis 25.

(37) In normal operation sensor 21 is to detect the actual motion of the motor axis 25 and to send its output signal to the control unit 18. Thus, exact positioning of the workpiece arm (2) is possible by analyzing the output signals of all encoders being attached, one to each motor axis (which is not limited to any number of axes, but including as many as necessary, such as 5 encoders and respective output signals for a 5/0 CAM device).

(38) In the reference position detection operation according to the present invention, it is further detected whether a control command instructing a motor to turn causes a respective encoder/sensor output signal. In the case that the tip of the tool (14) is already in contact with the contact or abutment element (28), the motor is not able to move further which means that no further sensor signal change can be detected. This is referred to as slip. When such a slip is detected, the reference position can be stored by the control unit (18).

(39) In an advantageous further embodiment, the drive motor is operated in normal operation in an open loop mode, which has advantages in speed. For reference determination, however, it is preferably operated in the closed-loop mode.

(40) The recorded reference position is also stored and can be used to calibrate the dental milling machine to the tool 14 as measured.

(41) The tool-specific code also allows the operating hours of the tool to be recorded by the control device 18. If the tool 14 is used again after only a few operating hours, it may not be required to re-measure it, i.e. to determine a new reference point. However, this can of course be done if required.

(42) Although this invention has been described above while making reference to the preferred examples, it is not limited thereto, but can be modified in many ways. In particular, the invention may be varied or modified in various ways without deviating from the core of the invention.

(43) For example, dimensioning or the geometric or material configuration of the position sensor can be varied or adapted to the respective structural requirements of the dental milling machine.